In recent years, attention has been paid to a micro-machine (MEMS: Micro Electro Mechanical Systems) element and a small device in which a MEMS element is incorporated. A basic characteristic of the MEMS element is that a driving body included as a mechanical structure is incorporated into a part of the element, and the driving body is driven electrically by an application of inter-electrodes coulomb's force and the like.
Since a micro-oscillation element formed by using a micro-machining technology based on a semiconductor process has such advantages that an area occupied by a device is small, a high Q-value can be obtained, and an integration with another semiconductor device is possible, a use as an intermediate frequency (IF) filter and a high frequency (RF) filter among wireless communication devices has been proposed by research laboratories including Michigan University (refer to Non-patent Reference 1).
FIG. 15 shows a schematic view of an oscillator, specifically a MEMS oscillator, constituting a high frequency filter described in Non-patent reference 1. This oscillation element 1 includes: an input-side wiring layer 4 and an output electrode 5 which are, for example, made of polycrystalline silicon, formed on a semiconductor substrate 2 through an insulation film 3, and a beam capable of oscillating made of polycrystalline silicon, for example, that is called a beam-type oscillation electrode 7 formed to face the output electrode 5 separated by a space 6. The oscillation electrode 7 straddles the output electrode 5 like a bridge and is connected to the input-side wiring layer 4 such that this beam is supported by anchor portions (support portions) 8 [8A, 8B] at the both ends. The oscillation electrode 7 becomes an input electrode. A gold (Au) film 9, for example, is formed at an end portion of the input-side wiring layer 4. In this oscillator 1, an input terminal t1 is led out from the gold (Au) film 9 of the input-side wiring layer 4 and an output terminal t2 is led out from the output electrode 5.
In this oscillator 1, a high frequency signal S1 is supplied to the oscillation electrode 7 through the input terminal t1 in a state where a DC bias voltage V1 is applied between the oscillation electrode 7 and the ground. Specifically, an input signal in which the DC bias voltage V1 and high frequency signal S1 are superimposed is supplied from the input terminal t1. When the high frequency signal S1 of an objective frequency is input, the oscillation electrode 7 having a natural oscillation frequency determined by a length L oscillates by electromagnetic power generated between the output electrode 5 and the oscillation electrode 7. With this oscillation, a high frequency signal corresponding to temporal change of capacitance between the output electrode 5 and the oscillation electrode 7 and the DC bias voltage is output from the output electrode 5 (therefore, from the output terminal t2). A signal corresponding to the natural oscillation frequency (resonant frequency) of the oscillation electrode 7 is output in a high frequency filter.
A resonant frequency of a micro-oscillator which has been proposed and examined does not exceed 200 MHz at maximum, and a high Q-value that is a characteristic of a micro-oscillator has not been provided in a GHz band frequency region with respect to a conventional filter of a GHz region based on a surface acoustic wave (SAW) or thin film acoustic wave (FBAR).
At present, typically there is such tendency that a resonance peak as an output signal becomes small in a high frequency region, and it is necessary to improve an S/N ratio of the resonance peak in order to obtain an excellent filter characteristic. In a disk-type oscillator according to a literature of Michigan University, a noise component of an output signal depends on a signal to be directly transmitted through parasitic capacitance C0 generated between the oscillation electrode 7 that becomes an input electrode and the output electrode 5. On the other hand, since a DC bias voltage exceeding 30V is necessary in order to obtain a sufficient output signal in the disk-type oscillator, a type of a beam structure using a both-ends-supported beam is desirable as a practical structure of the oscillation electrode.
However, in the case of the above-described oscillation element 1 of FIG. 15, since the space 6 between the oscillation electrode 7 and the output electrode 5 is small and facing areas of both electrodes 7 and 5 have a required size, the parasitic capacitance C0 between the oscillation electrode 7 that becomes the input electrode and the output electrode 5 becomes large. Accordingly, a ratio Z0/Zx of impedance Z0 of the parasitic capacitance C0 to impedance Zx of a resonance system (resistance Rx, inductance Lx, capacitance Cx) becomes small, so that the S/N ratio of the output signal becomes small. There is such a dilemma that the parasitic capacitance C0 becomes larger when it is attempted to make the output signal larger by making the space 6 between the oscillation electrode 7 and the output electrode 5 small.
[Non-patent Reference 1] C. T. —Nguyen, “Micromechanical components for miniaturized low-power communications (invited plenary)”, proceedings, 1999 IEEE MTT-S International Microwave Symposium RF MEMS Workshop, Jun. 18, 1999, pp, 48-77.
On the other hand, the applicants of the present invention have previously proposed a MEMS resonator aiming at a reduction of noise component in Japanese Patent Application No. 2003-11648. FIG. 14 shows a schematic view of the MEMS resonator. Basically, an oscillation electrode that becomes a beam to which a DC bias voltage is applied is disposed between input and output electrodes in order to reduce a noise component. As shown in FIG. 14, this MEMS resonator 11 includes: an input electrode 14 to input a high frequency signal and an output electrode 15 to output a high frequency signal formed at a required interval on a silicon semiconductor substrate 12 that has an insulation film on a surface, for example, and a beam, specifically an oscillation electrode 17, disposed to face those input and output electrodes 14 and 15 separated by space 16. The oscillation electrode 17 straddles the input and output electrodes 14 and 15 like a bridge and is integrally supported by support portions (what is called anchor portions) 19 [19A, 19B] at both ends to be connected to a wiring layer disposed outside the input and output electrodes 14 and 15.
In this MEMS resonator 11, a required DC bias voltage V1 is applied to the oscillation electrode 17, and a high frequency signal S1 is input into the input electrode 14. When a high frequency signal of an objective frequency is input, the oscillation electrode 17 resonates in a second-order oscillation mode by electromagnetic power generated between the oscillation electrode 17 and the input electrode 14 as shown in FIG. 14, for example. Since facing areas of the input and output electrodes 14 and 15 can be made small and an interval between the input and output electrodes 14 and 15 can be made large, parasitic capacitance C0 between the input and output electrodes 14 and 15 can be made small. Further, in order to obtain a large output signal, a distance of the space 16 between the oscillation electrode 17 and the input and output electrodes 14 and 15 can be reduced. Accordingly, a noise component of an output signal can be reduced in comparison to FIG. 15, and an S/N ratio can be improved.
Hereupon, when the space 16 between the oscillation electrode 17 and the input and output electrodes 14 and 15 is made smaller in order to obtain further larger output signal in the MEMS resonator 11 of FIG. 14, there is a possibility that the oscillation electrode 17 is adsorbed into the substrate 12 in a wet process during a manufacturing process, particularly in a process of removing a sacrifice layer. In addition, when applying to multi-order oscillation modes, it is difficult to select an oscillation mode of a desired order-number. Specifically, there is a possibility that the multi-order oscillation modes exists in a mixed manner.